1. Technical Field
The present invention relates to a volume hologram, a light source device, an illumination device, a monitor, and an image display device, particularly, a light source device supplying laser light.
2. Related Art
In recent years, using a laser light source that supplies laser light for use in a monitor or an image display device has been suggested.
Compared with a UHP lamp that is conventionally used as the light source device for a monitor or image display device, the light source device using the laser light source includes the advantages of high the color reproducibility, instantaneous lighting, long service life, and the like.
When the coherent laser light is irradiated onto a diffusing surface, an interference pattern called a speckle pattern, where light points and dark points are randomly distributed may be generated.
The speckle pattern is generated because the lights which are diffused by the diffusing surface at each point of the diffusing surface randomly interfere with each other.
When an image is displayed and a viewer watches the image with the speckle pattern, such as a garish flicker, generated from the image, the viewer may be annoyed.
Therefore, when using the laser light source, it is necessary to take countermeasures to prevent the speckle noise.
A countermeasure for preventing speckle noise, for example, as suggested in Published Japanese Translation No. 2004-503923 of PCT International Publication, is a technique where a plurality of emission elements is arrayed which supplies laser light whose wavelengths are different from each other.
Since the coherence length of laser light is substantially in inverse proportion to the spectral band width of laser light, the speckle noise is reduced and the coherence length is shortened by expanding the spectral band width.
Furthermore, by using an external resonator resonating laser light, a laser light source emits laser light with a high power.
As disclosed in Japanese Unexamined Patent Application, First Publication No. 2001-284718, for example, a technique using a volume hologram as the external resonator has been suggested.
By using the volume hologram, it is possible to produce laser light with narrow band width.
When an external resonator is included in the constitution, in order to oscillate laser light, it is necessary to use a resonance mirror having a high wavelength selectivity.
Even if a wide spectral bandwidth is obtained while emitting laser light from the emission element, the bandwidth of the laser light narrows in the external resonator.
When the bandwidth of laser light narrows in the external resonator, it is difficult to reduce speckle noise.
In contrast, in order to widen the wavelength-bandwidth of laser light when the selective wavelength-bandwidth of resonance mirror is widened, it is difficult to resonate the laser light, therefore the external resonator will be nonfunctional.
Furthermore, when a volume hologram is used, the wavelength-bandwidth of light can be on the very narrow sub nano-meter order.
When the bandwidth of light is narrowed as described above, though the characteristic of laser light is improved, there is disadvantage in that it is difficult to reduce speckle noise.
Additionally, in order to resonate laser light, whose wavelengths are different from each other, volume holograms corresponding to the emission elements which are disposed in an array may be used.
However, in this case, since alignment between all of the emission elements and all of the volume holograms is necessary, it is extremely difficult to assemble the light source device.
As described above, there is a problem in that it is difficult to reduce speckle noise in a conventional constitution where an external resonator is used.